Surface mineralized organic fibers and methods of making the same

ABSTRACT

A method of making a mineralized fiber having a fiber core and a calcium carbonate shell can include admixing fibers with green liquor and adding CaO to generate a causticization reaction that results in a calcium carbonate shell coating forming around the fibers.

CROSS-REFERENCE TO RELATED APPLICATION

The benefit of priority to U.S. Provisional Patent Application No. 63/083,528 filed Jun. 12, 2020 is hereby claimed and the disclosure is incorporated herein in its entirety.

BACKGROUND FIELD OF THE DISCLOSURE

The disclosure generally relates to surface mineralized organic fibers and methods of making the same, and more particularly to organic fibers coated with calcium carbonate and methods of coating organic fibers.

BRIEF DESCRIPTION OF RELATED TECHNOLOGY

Cellulose-calcium carbonate composite materials have been used in the paper industry as filler material. Conventional cellulose-calcium carbonate materials incorporate the calcium carbonate such that the essential fibrous nature of the cellulose component is maintained. It was generally recognized in the art that the fibrous nature of the composite materials was needed to better incorporate the composite materials in the fiber matrix of paper.

WO 97/01670 A1 relates to a filler used in papermaking and consisting primarily of calcium carbonate. The filler disclosed therein is porous aggregates of calcium carbonate particles, which are precipitated on the surface of fibers, e.g., cellulose fibers. The fillers described are based on the fact that calcium carbonate can be precipitated on the very fine fibers so that it adheres to the fibers. Among other things, this is due to the great fineness of the fibers, which have a length of max. 400 μm.

EP 0 930 345 A2 and EP 0 935 020 A1 disclose fillers similar to those described in WO 97/01670 A1, but wherein the calcium carbonate is not precipitated on the surface of the fibers but instead is mixed with them. These references teach that not only previously precipitated calcium carbonate may be used but also natural ground calcium carbonate may be used. The fibers have a fineness similar to that mentioned above, namely at most a P50 screen fraction, i.e., a maximum length of about 300 μm.

WO 98/35095 discloses a method for making paper, which comprises mixing an aqueous slurry of mineral filler with an aqueous slurry of wood fibers and the addition of flocculants wherein an essential portion of the filler is in the interior of the cellulose fibers. The filler and the flocculant are added to the pulp fibers independently of one another. The fillers are flocculated within the fibers and are kept in the interior, while the filler forms agglomerates outside of the fibers. The use of a binder which produces a uniform distribution of the filler on the surface of the fibers is not mentioned here either.

WO 99/14432 discloses a method for making paper by mixing anionic starch, carboxymethylcellulose or other polymeric binders together with a cationic inorganic or polymeric coagulant to form a thin cellulose pulp stock, and this suspension is then flocculated by means of an anionic swellable clay or other anionic retention aids.

SUMMARY

Thus, a number of mixtures and composites are known, which can be used to control certain properties of pigments and/or fillers. However, none of these discloses a laminate-type structure wherein a fiber is substantially coated and entrained by a calcium carbonate shell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph showing free fibers at the surface of tissue paper;

FIG. 2 is a graph showing panel scores for softness as a function of free fibers on a surface as disclosed in U.S. Pat. No. 4,300,981;

FIG. 3 is a schematic illustration of sodium and calcium loops in a kraft pulp mill;

FIG. 4 is a schematic illustration of an embodiment of a method of the disclosure;

FIG. 5 includes scanning electron microscopy (SEM) images of fibers before and after surface mineralization by a method in accordance with the disclosure

FIG. 6 include scanning electron microscopy (SEM) images of fibers before and after precipitation of calcium carbonate on the surface of the fibers by a conventional method;

FIG. 7 is a graph comparing bulk properties as a function of the content surface-mineralized organic fiber in a tissue product;

FIG. 8A is a scanning electron microscopy (SEM) image of a handsheet made by a conventional process using standard PCC filler as an additive;

FIG. 8B is a scanning electron microscopy (SEM) image of a handsheet made using a 50/50 blend of standard pulp fiber and mineralized fiber in accordance with the disclosure;

FIG. 9 is a graph showing the sheet bulk for a sheet prepared with mineralized fibers as compared to the starting (unmineralized) fibers; and

FIG. 10 is a graph showing the mineralization efficiency as a function of specific surface area of the starting fiber.

DETAILED DESCRIPTION

Provided herein are mineralized organic fibers. The organic fibers can be coated and entrained with an outer calcium carbonate coating. The organic fibers can include or can be cellulosic fibers. Any pulp fiber can be included. For example, the pulp fiber can be Eucalyptus, Birch, Acacia, Aspen, Pine, Spruce, mixed tropical hardwood, old corrugated cardboard recycled fiber, and combinations thereof.

The outer calcium carbonate coating is provided as a shell around a cellulosic fiber core. Embodiments of the disclosure can also include calcium carbonate infiltration into the cellulosic fiber core as well as in a shell structure. Surface strands of the fibers can be entrained or coated with the calcium carbonate. The fiber can have a hollow interior. Mineralization in accordance with the disclosure can result in calcium carbonate infiltrating into the hollow interior as well as coating surface strands and forming a shell around the fiber core. Entrainment of the organic fibers as disclosed herein has advantageously been found to increase bending stiffness of the fibers, which can lead to improved performance in a variety of applications, such as in papermaking and particularly paper intended for use in printing and hygienic applications. Such entrainment and/or infiltration of the calcium carbonate into the fiber cannot be achieved when adding calcium carbonate as a filler separately in the papermaking process. Surprisingly, it has been found that the surface-mineralized organic fibers can be incorporated into the fiber paper matrix despite the increase bending stiffness and that certain properties may be enhanced. For example, in tissue paper products incorporation of the surface-mineralized organic fibers of the disclosure can allow for improved softness. This is contrary to conventional understanding that the fibrous nature of the organic fiber filler was needed for proper incorporation into the fiber matrix of paper.

Incorporation of surface-mineralized organic fibers in accordance with the disclosure into a tissue paper product can improve bulk properties. Referring to FIG. 7 , illustrates the effect of the degree of mineralization. It was found that a degree of mineralization of 60% (that is incorporation of 60% of the surface-mineralized organic fibers with 40% standard pulp fiber (referenced as MinFib in FIG. 7 )) significantly improved bulk as compared to a control paper having no surface-mineralized organic fibers. It was surprisingly found that highest bulk properties were found with correspondingly high total ash level in the sheet, which is contrary to conventional expectation in the art. FIG. 9 illustrates the bulk improvement resulting from mineralized fibers of the disclosure. At 10% and 25% mineralization as compared to the fiber alone. It can be advantageous to have a degree of mineralization of greater than 12% (i.e., greater than 12% surface-mineralized organic fibers based on the total fiber content). For example, embodiments of the disclosure can have tissue products made using a degree of mineralization of about 20% to about 70% by inclusion of about 20% to about 70% surface-mineralized organic fibers of the total fiber content.

It has been observed that the softness of hygienic papers such as facial and bathroom tissues is in large part correlated with free fibers protruding from the paper surface. FIG. 1 shows a photomicrograph of free fibers extending from the surface of piece of tissue paper. U.S. Pat. No. 4,300,981 discloses the results of panel testing of tissue softness. The test panel rated various tissues as “softer” as the number of free fibers at the surface increased.

It has been observed that the surface-mineralized fibers of the disclosure can be used to increase the number of free fibers at the surface of tissue papers. While not wishing to be bound by any particular theory, it is believed that enhanced stiffness of the individual mineralized fibers prevent non-mineralized fibers from forming the natural hydrogen bonds that give paper its structural integrity and instead be provided as free fibers. Although it is counter-intuitive, in this manner the stiffer mineralized fibers lend themselves to overall increased softness in the bulk tissue sheet. Referring to FIGS. 8A and 8B, it can be seen that increased free fiber is achieved in a tissue product having the surface mineralized fibers as compared to precipitated calcium carbonate added to the tissue product by conventional methods. In the embodiments of FIGS. 8A and 8B, the fiber used was Eucalyptus. In the conventionally prepared sample of FIG. 8A, the precipitated calcium carbonate was present in an amount of 20% and the fiber was present in an amount of 80% based on the total weight of the composition. The conventional sample of FIG. 8A had an ash content of 10.7%. By comparison, the composition in accordance with the disclosure (FIG. 8B) included 50% surface-mineralized fiber and 50% Eucalyptus native fiber. The surface mineralized fiber was prepared by the methods of the disclosure to have a calcium carbonate shell around a Eucalyptus fiber as well as infiltration of the calcium carbonate into fiber an entrainment of the fibers with the calcium carbonate. The ash content of this composition was 32.4%.

METHOD OF MAKING

Methods of making surface-mineralized organic fibers can include admixing fibers with sodium carbonate to form a fiber slurry. In embodiments, the fibers can be an aqueous slurry and the sodium carbonate source can be a green liquor. In embodiments, the sodium carbonate can be provided as an aqueous solution. In embodiments, the sodium carbonate source can be a mixture of Na₂CO₃/Na₂S/H₂O. To the fiber slurry, dry CaO is added until the stoichiometric amount of CaO is less than the stoichiometric amount of sodium carbonate (Na₂CO₃) required for the CaO to fully react with the sodium carbonate. The resulting mixture is mixed for sufficient time for the causticization reaction (1) to be essentially complete. When green liquor or other mixture of Na₂CO₃/Na₂S/H₂O is used, the causticization reaction is as follows.

CaO+Na₂CO₃/Na₂S/H₂O

CaCO₃+surface mineralized fiber+2NaOH/Na₂S/H₂O  (1)

The causticization reaction results in fibers coated with the calcium carbonate (surface mineralized fibers), free calcium carbonate, and a mixture of NaOH/Na₂S/H₂O, which is the composition of white liquor. In embodiments in which sodium carbonate is used as opposed to a green liquor, NaOH results as the by-product. The method can further include separating the surface mineralized fiber from sodium hydroxide or the white liquor and the excess calcium carbonate. This can be done, for example, by passing the resulting mixture from the causticization reaction over a screen. The collected surface mineralized fiber can then be washed with water to separate any remaining excess calcium carbonate. The washed surface mineralized fiber can then be collected. As detailed below, in embodiments, the method can be performed as part of the kraft process of a pulp mill, which can allow for the white liquor, excess calcium carbonate, and weak wash to be recycled into the kraft process.

In embodiments, the method can further include scraping a portion of the calcium carbonate shell from the surface of the surface-mineralized organic fibers. Without intending to be bound by theory, it is believed that scraping of a portion of the calcium carbonate shell can improve fiber to fiber bonding when used as a filler. Scraping can be performed to remove a portion of the calcium carbonate without adversely affecting the bending stiffness of the surface-mineralized fiber.

Methods of making surface-mineralized organic fibers disclosed herein can allow for rapid formation of the mineralized fiber composite to limit losses of unreacted soluble ions by recycling them back into the process for reuse. As is well understood in the art, precipitation of precipitated calcium carbonate (PCC) in the presence of cellulose pulp fiber is an inherently ineffective process because the viscosity of the aqueous slurry of pulp fiber is very high if the fiber is present in an amount greater than about 1 wt %. Such high viscosity detrimentally prevents calcium (Ca²⁺) ions that are added to the slurry from freely coming into contact with dissolved carbonate (CO₃ ²⁻) ions; such contact being necessary for calcium carbonate to precipitate. This slows the kinetics of the reaction and causes throughput in the reactor to become unacceptably low. Further, since the amount of fiber in the reactor determines the amount of mineral that can be attached and coat its surface, the useful amount of the composite material present in the reactor at the completion of the reaction will necessarily be low thus leading to low output of product.

It has advantageously been found that control over process parameters can allow for coating of the calcium carbonate to form a shell structures as opposed to conventional processes in which calcium carbonate is merely attached to the surfaces. Conventional processes are inefficient at coating fibers and typically result in only intermittent surface attachment of calcium carbonate on the fiber. This is generally considered acceptable for conventional processes because it was believed that it would be disadvantageous to affect the bending stiffness of the fiber with the calcium carbonate. In contrast, it has been found that methods of increasing the content of calcium carbonate to form a shell structure that stiffens the fiber can be advantageous. Embodiments of the method result in calcium carbonate forming as a shell structure on the exterior surface of the fiber, as well as infiltrating into the interior surface of the fiber.

Embodiments of methods of the disclosure take advantage of processes normally done in every pulp mill that utilizes the kraft pulping process. Referring to FIG. 3 , in the kraft process, black liquor coming from the pulp digester is converted to green liquor containing a high concentration of dissolved sodium carbonate (Na₂CO₃). The green liquor is then converted to white liquor by causticizing it with quicklime (CaO) so that the sodium carbonate is substantially converted to sodium hydroxide (NaOH, caustic soda). Along with the caustic soda, calcium carbonate is also precipitated. In a kraft pulp mill, this calcium carbonate is called lime mud.

The methods of the disclosure can utilize a portion of pulp fiber is taken from pulp operation (not shown in FIG. 3 ) that follows the Cooking & Washing step in FIG. 3 and combines it with a portion of green liquor to form a slurry of pulp in green liquor. In embodiments, the fiber can be refined. For example, the fiber can be taken from a pulp refining operation. For example, the fiber can be refined to a level of 20 SR to 90 SR. To this slurry is added an amount of quicklime (CaO) in an amount that is less than the stoichiometric amount needed to completely react with the sodium carbonate dissolved in the green liquor. This is done to ensure that the entire amount of calcium introduced with the quicklime is reacted to form calcium carbonate which precipitates and surrounds the fibers present in the slurry. The amount of fiber introduced to the green liquor and the amount of quicklime that is fed into the green liquor-fiber slurry are chosen so that the fibers are completely entrained within the calcium carbonate mineral. The quicklime can be added, for example, by a screw-feeder.

Following the formation of the fiber-mineral composite the composite is passed over a screen and washed with water or weak wash from the pulping operation in order to remove any excess green liquor and any calcium carbonate that is not bound to the fiber. Following the washing step, solid calcium carbonate is separated from the alkaline liquor and sent on to the mud kiln shown in FIG. 3 . The alkaline liquor portion is sent to part of the process where Green Liquor is formed coming out of the Black Liquor Recovery Boiler. Referring to FIG. 4 , in this manner, all reactants and products of the process of the disclosure are recovered leading to an operational efficiency that would not otherwise be possible.

It has been found that the specific surface area of the starting fibers affected the mineralization efficiency, with higher efficiency being achieved with higher specific surface area materials. As used herein, mineralization efficiency refers to the percent of calcium carbonate that is present on or in the fiber of the total calcium carbonate produced in the reaction. Calcium carbonate not present or in the fiber remains loose in the reaction and can be washed away and optionally recycled. FIG. 10 illustrates the increased mineralization efficiency resulting from using a starting fiber with a specific surface area of 40 m²/g verses 4 m²/g. The efficiency increased significantly from 3.3% efficiency for the low specific surface area fiber to 95% for the high specific surface area material. Increased mineralization efficiency can allow for reduced amounts of calcium carbonate to be wasted in the processes. Further, high efficiency was found to result in increased amount of calcium carbonate present on or in the fiber, which beneficially increased bending stiffness even as compared to the mineralized low specific surface area fibers.

Native fibers used in the process of mineralization can have a specific surface area of about 2 m²/g to about 80 m²/g, about 2 m²/g to about 10 m²/g, about 15 m²/g to about 60 m²/g, about 40 m²/g to about 80 m²/g. Other suitable specific surface areas include, for example, about 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78 and 80 m²/g and values there between and ranges having endpoints defined by these values.

It has been found that incorporation of the methods of the disclosure does not adversely affect or otherwise disrupt the kraft process. In particular, the white liquor that is removed after formation of the fiber-mineral composite and the weak wash that is created when unbound calcium carbonate is washed away from the fiber-mineral composite is substantially the same composition as white liquor and weak wash present in the kraft process normally.

EXAMPLE

A mixture of sodium carbonate (Na₂CO₃) and sodium hydroxide (NaOH) was prepared. This simulated green liquor typically found in kraft pulp mills with the exception that sodium sulfide (Na₂S), which is a component of typical kraft green liquors was not used in the lab-scale experiment due to its potential to generate hydrogen sulfide (H₂S), an extremely poisonous gas. The simulated green liquor was prepared by combining 72 g of dry NaOH pellets, 207 g of dry, technical grade, sodium carbonate powder and 1375 g of tap water using a mechanical agitator until a solution was formed.

1.7 g of air dry unrefined cellulose fiber was combined with 146.1 g of tap water and mixed under mechanical agitation to form a slurry. The fiber slurry and simulated green liquor solution were then combined in a 4-liter glass reactor fitted with mechanical mixing blades and an electric heating mantle and heated to about 98° C. while under agitation at 1,000 rpm.

100 g of a U.S. commercial quicklime were added to the reactor over a time of 100 minutes with stirring and maintaining a temperature near 100° C. After the quicklime had been added, agitation at temperature was continued for 50 minutes.

Throughout the quicklime addition and subsequent mixing, considerable evaporation is evident. Following completion of the reaction 1,089 g of material was recovered from the reactor as a slurry of the surface mineralized fiber, white liquor and calcium carbonate. This was passed over a 230 mesh screen to recover 104 g of material on the screen, with the white liquor passing through the screen being collected and saved.

The material remaining on the screen was washed with 12,180 g of tap water before being collected. FIG. 5 illustrates the resulting surface mineralized fibers having the fiber core and calcium carbonate shell.

COMPARATIVE EXAMPLE

A conventional process of precipitated calcium carbonate on a fiber is as follows. FIG. 6 illustrates the result of the conventional process. Referring to FIGS. 5 and 6 , as compared to the resulting fibers of the disclosure, the calcium carbonate present in the conventional fiber having calcium carbonate precipitated thereon is much more sporadically positioned along the fiber and does not form a shell structure that would affect the bending stiffness as with the fibers of the disclosure.

As with conventional processes, the conventional fibers having the precipitated calcium carbonate of this example were made by adding 65 liters of water at a temperature of 21° C. water to a mortar. 12.5 dry kilograms of CaO was then added and mixing was continued for 10 minutes to form a milk-of-lime [Ca(OH)₂] slurry. Following mixing, the Ca(OH)₂ slurry was poured over a No. 200 mesh sieve to remove oversized non-reactive components. The concentration of the Ca(OH)₂ slurry passing through the sieve was determined, by titration, to be 0.22 g/ml.

Twenty (20) liters of 45° C. water were added to a 100 liter reactor, which was fitted with two agitation impellers rotating at 250 rpm. To the water in the reactor were added 699.34 grams of 27.6% solids refined Eucalyptus pulp (192.8 air-dry grams equivalent). This was 1% by wt. dry fiber on dry PCC yield after which the speed of the impellers was increased to 450 rpm. Using a 20% CO₂ in air gas mixture, CO₂ was bubbled into the contents of the reactor at a rate of 0.49 standard cubic feet per minute (SCFM), with a corresponding air flow rate of 1.95 SCFM. Following the start of CO₂ gassing, the milk-of-lime from Step 3 was added to the reactor using a peristaltic pump so as to maintain an electrical conductivity of 3.5 mS/cm.

Simultaneous addition of milk-of-lime and CO₂ were continued until all the milk-of-lime had been added. At that point, CO₂ addition was continued until a pH of 7.0 was measured for the contents of the reactor. Addition of CO₂ was then stopped and the contents of the reactor were screened through a No. 200 mesh sieve to separate any free calcium carbonate from mineralized pulp fiber.

The mineralized pulp fibers remaining on the No. 200 mesh sieve were washed with large volumes of water to remove all unattached calcium carbonate.

A sample of mineralized fiber was taken from the sieve. Ashing at 525° C. for four hours was used to determine that the mineralized fiber composite contained 60.3% calcium carbonate.

The technical information set out herein may in some respects go beyond the disclosure of the invention, which is defined exclusively by the appended claims. The additional technical information is provided to place the actual invention in a broader technical context and to illustrate possible related technical developments. Such additional technical information which does not fall within the scope of the appended claims, is not part of the invention.

While particular embodiments of the present invention have been shown and described in detail, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects. Therefore, the aim is to cover all such changes and modifications as fall within the true spirit and scope of the invention. The matters set forth in the foregoing description and accompanying drawings are offered by way of illustration only and not as limitations. The actual scope of the invention is to be defined by the subsequent claims when viewed in their proper perspective based on the prior art. 

1. A mineralized fiber comprising: a fiber defining a core of the surface mineralized fiber, and calcium carbonate surrounding the core and infiltrated into the fiber, wherein the calcium carbonate increases the bending stiffness of the fiber.
 2. The mineralized fiber of claim 1, wherein the fiber has a hollow interior and the calcium carbonate infiltrates into and the hollow interior.
 3. The mineralized fiber of claim 1, wherein the calcium carbonate infiltrates into the fiber such that at least a portion of surface strands of the fiber are encapsulated by calcium carbonate.
 4. The mineralized fiber of claim 1, wherein the fiber has a specific surface area of about 2 m²/g to about 80 m²/g.
 5. The mineralized fiber of claim 1, wherein the fibers are pulp from one or more of Eucalyptus, Birch, Acacia, Aspen, Pine, Spruce, and old corrugated cardboard recycled fiber.
 6. A method of making a mineralized fiber, comprising: admixing fibers with Na₂CO₃ form a fiber slurry; adding dry CaO to the fiber slurry in an amount such that the stoichiometric amount of CaO is less than the stoichiometric amount of sodium carbonate presented in the fiber slurry and admixing for a time sufficient to allow the CaO to react with the sodium carbonate in a causticization reaction, thereby forming the surface mineralized fibers comprising a fiber core with calcium carbonate surrounding the core and infiltrating into the fiber.
 7. The method of claim 6, further comprising passing the surface mineralized fibers resulting with the admixture of the dry CaO with the fiber slurry over a screen to separate the surface mineralized fibers from excess calcium carbonate and NaOH produced during the causticization reaction and washing the separated surface mineralized fibers with water to remove excess calcium carbonate.
 8. The method of claim 7, wherein the screen is a 230 mesh screen.
 9. The method of claim 6, wherein the fibers are pulp from one or more of Eucalyptus, Birch, Acacia, Aspen, Pine, Spruce, and old corrugated cardboard recycled fiber
 10. The method of claim 6, wherein the fibers are pulp from a kraft process and the Na₂CO₃ is provided as green liquor (Na₂CO₃/Na₂S/H₂O) from the kraft process.
 11. The method of claim 10, further comprising passing the surface mineralized fibers resulting with the admixture of the dry CaO with the fiber slurry over a screen to separate the surface mineralized fibers from excess calcium carbonate and NaOH/Na₂S/H₂O produced during the causticization reaction and washing the separated surface mineralized fibers with water to remove excess calcium carbonate, and recycling the NaOH/Na₂S/H₂O, excess calcium carbonate, and collected wash solution into the kraft process.
 12. The method of claim 11, wherein the screen is a 230 mesh screen.
 13. The method of claim 6, wherein the fibers are provided as an aqueous slurry.
 14. The method of claim 6, wherein the fibers and the NaOH/Na₂S/H₂O are admixed while heating to a temperature of about 70° C. to about 105° C.
 15. The method of claim 6, wherein the dry CaO and the fiber slurry are admixed while maintaining a temperature of about 70° C. to about 105° C.
 16. The method of claim 6, wherein the fiber is a refined fiber refined to a level of 20 SR to 90 SR. 